Electron transfer mechanisms underlying magnetite-mediated nitrate removal in electro-assisted mixotrophic denitrification for recirculating mariculture wastewater treatment.

Efficient nitrate removal from recirculating mariculture wastewater is constrained by low carbon-to-nitrogen ratio and high nitrate concentrations. Although mixotrophic denitrification can integrate organic- and sulfur-driven pathways, its performance is still limited by inefficient electron transfer and poorly coordinated sulfur and nitrogen metabolism. In this study, magnetite was immobilized on a carbon-felt anode, and an electro-assisted mixotrophic denitrification system was constructed to regulate electron flow. At a shortened hydraulic retention time of 12 h, more than 99% NOx--N removal was maintained in the magnetite-modified system, while sulfate accumulation was reduced. Elemental sulfur accumulation of 168.7 mg/L was achieved, corresponding to 2.3 and 1.9 times the levels measured in the non-electrode and bare-anode system, respectively, indicating that sulfur conversion was redirected toward elemental sulfur. Following anode modification, increased redox responses associated with flavin- and cytochrome c-like components in extracellular polymeric substances were observed. Intracellular electron transport was concurrently intensified, as evidenced by a 61.5% increase in electron transport system activity and enhanced nitrate and nitrite reductase activities. Distinct microbial community restructuring was observed, while coordinated changes in functional profiles related to sulfur oxidation (sox and fccAB), electron transport, and denitrification (napAB, nirK, and nosZ) were identified. Linkages among sulfur oxidation, respiratory electron transport, and nitrate/nitrite reduction were further revealed by correlation analysis. These findings improve understanding of electron-transfer regulation and its role in nitrate removal.